Throttle Position Sensor: Explained

The throttle position sensor is one of the most important sensors in automotive electronic control systems, mainly used in engine electronic fuel injection systems and electronically controlled automatic transmission systems. The throttle position sensor is installed at one end of the throttle shaft on the throttle body, detects or monitors the size and speed of the throttle opening, converts the position signal into an electrical signal, and then inputs it to the electronic control unit. It is used to discriminate various working conditions of the engine, so as to control different fuel injection quantities and ignition timings. On vehicles with electronically-controlled automatic transmissions, the throttle position sensor signal is the primary signal for transmission shifts and torque converter lockup.

Throttle Position Sensor Functions

What does a throttle position sensor do? The throttle position sensor is also known as a throttle opening sensor or throttle switch. Its main function is to detect whether the engine is in an idling condition or a load condition and whether it is an acceleration condition or a deceleration condition. It is essentially a variable resistor and several switches, which are installed on the throttle body. The shape and internal structure are shown in the figure below.

 

The shaft of the resistor is linked to the throttle valve, and it has two contacts: a fully open contact and an idle contact. When the throttle valve is at the idle speed position, the idle speed contact is closed, and the idle speed condition signal is output to the computer; when the throttle valve is at other positions, the idle speed contact is opened, and the voltage signal at different rotation angles relative to the throttle valve is output, and the computer is based on the signal. The voltage value identifies the load of the engine; according to the change rate of the signal voltage within a certain period of time, it identifies whether it is an acceleration condition or a deceleration condition. The computer revises the fuel injection quantity according to these working conditions information or carries on the fuel cut-off control.

Bad Throttle Position Sensor Symptom

In the following part, Easybom will elaborate on “what happens when throttle position sensor goes bad” or “symptoms of a bad throttle position sensor“.

The throttle position sensor is a friction part and a vulnerable part. If the throttle position sensor is damaged, there will be two problems:

One is that the idle speed of the car is unstable because the sensor detects the real-time condition of the engine through two contacts. Once there is a problem, it will lead to very unstable idle speed.

The second is not being able to start the car, even if it is started, it will often turn off.

If these two situations are often encountered, it means that the throttle position sensor is broken, and the owner needs to replace the sensor in time to avoid other unnecessary effects on their car.

 

Throttle sensor failure case analysis (1)

Instantaneous surge occurs when the engine speed reaches 3000 rpm

Phenomenon: For a car, the customer reported that the car often experienced surges, and every time a surge occurred, the throttle valve (accelerator pedal) was almost in the same position, and the failure of fuel consumption increased and power decreased at the same time.

Analyze:

1. Throttle position sensor failure
2. The crankshaft position sensoris faulty and the signal is unstable
3. The ignition system is faulty, resulting in a lack of fire at a certain time
4. Occasional failure of the airflow meter

Diagnosis:

1. Retrieve the fault code and show that the mixture ratio is not good. It can be inferred that the failure is necessarily related to the opening of the throttle valve. Detect the throttle position sensor with an oscilloscope, and it shows that its waveform shows a gentle downward trend with the increase of the throttle valve opening, and the orientation is smooth without burrs, indicating that the throttle valve position sensor is normal.
2. Because of another failure phenomenon, the fuel consumption increases and the power decreases. Then check the airflow meter and oxygen sensor. The air mass flow rate is 4.8g/s when the idle speed is measured, and the signal voltage of the oxygen sensor shows about 0.8V. In order to verify the quality of the O2S, unplug a vacuum tube on the intake manifold and the engine starts at high idle speed, the O2S signal drops from 0.8V to 0.2V, indicating normal. During the idling operation, the airflow has been oscillating slightly at 4.8g/s. After unplugging the plug of the airflow meter, the test is started again, and the fault disappears. Troubleshoot after replacing the airflow meter.

To solve the puzzle:

When it is suspected that a certain sensor is faulty, the method of unplugging the sensor plug (the crankshaft position sensor cannot be pulled out, otherwise the vehicle cannot be started) can be used for testing. When a plug is unplugged, the control of the ECU will enter the backup program and replace it with the stored or other signal values. If the fault disappears after the plug is unplugged, it means that the fault is related to the sensor.

Throttle sensor failure case analysis (2)

A Guangzhou Honda Accord 2.4 car was flooded due to water wading. The water entered the engine cylinder and the connecting rod was bent. The owner overhauled the engine in a repair factory. When leaving the factory, the repairman found that the shift lever could not be hung when it was in the P position. After entering the D position, the vehicle could not be driven and was towed into the special maintenance station for maintenance.

The fault diagnosis instrument HDS was used for detection, and it was found that the signal of the throttle position sensor was abnormal. The normal opening should be about 9% under the idling condition, but the signal of the throttle position sensor of the car showed 37%, which was very different from the normal signal.

Next, test the throttle position sensor (as shown in the attached picture). Under normal circumstances, the resistance values between the pins of the throttle position sensor are as follows when the throttle position sensor is fully closed: 1-3 feet are 5.70kΩ, 1-2 feet are 5.66kΩ, 2-3 feet are 1.06kΩ. In the detection, it was found that the corresponding resistance values of the car’s throttle position sensor are as follows: 1-3 feet are 9.89kΩ, 1-2 feet are 9.98kΩ, and 2-3 feet are 1.60kΩ.

 

Due to the water entering the engine air intake system, the water enters the throttle position sensor from the throttle shaft, causing the resistance of the sliding potentiometer to increase, and the poor signal causes the automatic transmission shifting mode to be disordered. Therefore, in order to protect the automatic transmission mechanism, the PCM sends a signal to control the key interlock solenoid valve through the multi-channel control device to lock the shift lever in the P position. Do not let the vehicle continue to drive to protect the engine and automatic transmission assembly.

Since the throttle position sensor cannot be replaced separately, only the throttle body assembly can be replaced. After the replacement, the fault is eliminated.

Causes of Throttle Position Sensor Failure

1. Corrosion of the joint, poor contact, low voltage, or non-conduction, cause the engine to easily stall or difficult to start.
2. The initial position is inaccurate, and the output voltage is high or low, resulting in unstable idle speed, high or low.
3. The coating fails, and the resistance change causes the output voltage to be inaccurate, the vehicle starts to rush, and the acceleration is poor.
4. Too much carbon deposits at the throttle valve, and the closing is not strict, which affects the output voltage value and flow value, resulting in inaccurate air-fuel ratio and unstable idle speed.

Throttle Position Sensor Installation Location

Where is the throttle position sensor located? The throttle position sensor is installed on the throttle body. With the change of the throttle opening and the rotation of the throttle shaft, the brush in the sensor slides or the guide cam rotates, and the angle signal of the throttle opening is converted into an electrical signal and transmitted to the ECU. Vehicles equipped with automatic transmissions usually use a linear output throttle position sensor.

Throttle Position Sensor Adjustment Method

(1) Loosen the two fixing screws of the throttle position sensor.

(2) Insert a thickness gauge with a thickness of 0.35mm between the throttle limit screw and the limit rod, and use a multimeter to measure the continuity of the idle switch.

(3) Turn the throttle position sensor counterclockwise to disconnect the idle switch contact, and then slowly turn the throttle position sensor clockwise until the idle switch is closed.

(4) Tighten the two fixing screws of the throttle position sensor.

(5) Insert thickness gauges of 0, 30mm, and 0.40mm between the throttle limit screw and limit rod respectively, and measure the conduction of the idle switch at the same time. When the thickness gauge is 0.30mm, the idle speed switch should be turned on; when the thickness gauge is 0.40mm, the idle speed switch should be turned off. Otherwise, the throttle position sensor should be readjusted.